Portable Medical Device System

ABSTRACT

The present invention provides an apparatus comprising a wound dressing connected to a fluid container via a pump, wherein the wound dressing is in communication with a mechanical pressure control valve, the fluid container is provided with an inlet and an outlet. Also provided are ( ) flexible fluid containers comprising of at least two layers of film with an integrated vent, (ii) wound dressings and (i) a multi-compartment wound fluid container comprising at least two internal compartments and provided with an outlet and an inlet, in which the container comprises a microporous fluid separator which divides the at least two internal compartments, wherein the microporous fluid separator permits gas flow between the compartments and prevents fluid flow to the outlet of the container. Other apparatus provided comprises a means for detecting the level of fluid within a multi-compartment wound fluid container as described. The invention also provides a system for applying a sub-atmospheric pressure to a wound dressing on a patient using devices and apparatus of the invention and methods of treatment of wounds using such apparatus, devices and systems of the invention.

The present invention relates to apparatus for Negative Pressure WoundTherapy (NPWT) in patients suffering from exposed wounds.

Negative Pressure Wound Therapy, Reduced Pressure Therapy orSub-Atmospheric Pressure Therapy is used to treat hard to heal woundsand works on the principle of applying a sub-atmospheric pressure(normally between 50 mmHg and 200 mmHg gauge pressure) to a porousdressing situated at the wound site. Typically the porous dressing wouldbe held in place with a partially occlusive adhesive film and a tubeconnects the dressing assembly to a rigid fluid container fitted to thepressure generating device, which consists of a low flow vacuum pump, areceptacle for the fluid container, a control system and a moulding tohouse the various components. In a typical device in clinical use acontrol system energises the pump which in turn evacuates the dressingand draws air and fluid out of the wound dressing site via the tube intothe rigid container. When the air has been evacuated a negative pressureis established at the wound site which is communicated back to thecontainer inside the pump unit housing via the tubing, this in turn iscommunicated to the control system within the unit housing. The controlsystem regulates the vacuum pump to maintain the required set pressure.Examples of NPWT systems in clinical use are the InfoVAC® manufacturedby KCI of San Antonio, Tex., USA and Renasys® system manufactured bySmith and Nephew of Hull, UK.

Conventional means of collecting wound fluid in a Negative PressureWound therapy system comprises of a vacuum port and a fluid inlet portincorporated into a rigid sealed plastic container. A negative pressureis generated within the rigid container by evacuating the air within thecontainer via the vacuum port where fluid is then drawn into thecontainer. A hydrophobic filter prevents fluid being drawn into thevacuum port, see for example WO 2000/061206. A limitation of thistechnique is that the container has to be sufficiently rigid towithstand the negative pressure generated within the container,typically up to 300 mmHg. Additionally the container occupies a fixedvolume in both the unfilled and filled condition which could be up to1000 cm³. This has implications for material cost, storage andpotentially prevents the use of the therapy in a portable applicationdue to size and patient comfort. In addition to controlling pressure byvarying the pump speed one method is to provide a proportional valvemechanism between the inlet and outlet ports of the vacuum source. Thisvalve can move between the open and a closed position and between thevacuum source and the wound fluid container to regulate the pressure atthe wound site. The valve mechanism is controlled by a control unit thatreceives pressure signals from a transducer. An example of this approachis described in US 2010/0204663. The disadvantage of this technique isthat it requires electronic circuitry to measure and provide signals tocontrol a proportional valve. Furthermore, by controlling pressurewithin the vacuum source housing there are inevitable pressure drops andhydrostatic head effects that can alter the accuracy of the pressuredelivered at the wound site.

In addition to a standard diaphragm air vacuum, a peristaltic pump orother type of peristaltic pump can be used to generate a negativepressure. The advantage of using a peristaltic pump to remove fluid isthat the fluid can be contained within the tube and a suitablecontainer.

A pressure sensor can be provided at the wound site and a signal sentback to the unit controlling the pump via an electrical wire in thefluid tubing. Depending on the signal from the pressure sensor then thedrive voltage to the pump can be altered. The use of a peristaltic pumpfor applying a vacuum pressure to a wound is described in includes US2007/0055209. The disadvantage of using a peristaltic pump in this wayis the inability to provide effective pressure control at the woundsite, reducing the speed of the pump does not effectively reduce thepressure at the wound site because of the sealed nature of theperistaltic pump system. Without any opening to the atmosphere, thepressure at the wound can be maintained for a long period of time evenwhen the pump is switched off because it is a sealed system.Additionally providing an electronic sensor at the wound site providesadditional complications and potential safety concerns especially in aportable system for the home.

An alternative means of collecting fluid in an expandable containerconsists of utilising a wicking element that is attached to a bodyincorporating the inlet and outlet connections. Fluid is drawn into thebody via the fluid port and a wicking component transfers the fluid intothe expandable portion of the container, see for example WO2009/106895.

Negative Pressure Wound Therapy (NPWT) was first introduced intoclinical practice over 15 years ago. Since then the body of evidence forits use in both the hospital and community setting has grown rapidly.The technology was originally developed for the treatment of complexwounds within the hospital environment. Recent studies have identifiedthat the use of NPWT in the community can offer significant advantagesfrom both an economic and patient benefit perspective. The economic andsocial benefits are derived from the possibility to deliver an effectivetherapy into the community at an economic price to the healthcareprovider. If a patient is discharged early with a complex wound and itstarts to regress in the community the cost can quickly escalate withthe additional nursing costs involved and potential readmission. Costsincrease dramatically if the wound becomes infected and high dependencyor intensive care treatment is involved.

In addition to the economic cost there is a large human cost associatedwith living with complex wounds. However traditional NPWT systems arenot ideally suited for use in the home due to the size of the unitrequired to house the vacuum pump, control system and the receptaclerequired for the fluid container. Additionally hospital style NPWTsystems also require the disposal of the components that are in directcontact with the wound fluid. This typically includes the dressingcomponents, PVC tubing and the fluid container systems which can rangein size from 300 ml to 1000 ml.

Some portable systems have been developed for use in the community suchas the Pico™ system manufactured by. Smith & Nephew which consists of adressing system and a small pump system and where the entire system isdisposed after a course of treatment. Typically portable systems eitherconsist of a traditional style rigid fluid container with a smallercapacity to ensure the overall system size is reduced or a passivesuper-absorbent dressing that absorbs the wound exudate.

Monitoring and control of pressure at the wound site is achieved inseveral different ways by commercial devices currently on the market.KCI (San Antonio, Tex.) utilises a technology called T.R.A.C™. Thisconsists of a multi-lumen tube consisting of a fluid path and sensinglumens which transmit the pressure at the wound site to a pressuresensing circuit in the product housing.

Other commercial devices utilise a system that measures pressure insidethe fluid container on the wound side of the protective hydrophobicfilter. The pressure feedback control electronics then regulates thepump power to meet the target pressure.

Other commercial systems provide a fixed bleed within the pneumaticsystem to stimulate flow and aid pressure control

With regard to the problem associated with orientation of the canisteror movement causing the filter to occlude before the container is full,solutions have been tried that include the addition of a super absorbentgel within the container that partially solidifies the fluid to preventwetting of the filter. Other designs have been tried that involvecompartmentalising of the rigid canister structure that providesmulti-facets of the filter in different planes.

Most commercial devices utilise a diaphragm pump that normally consistsof a brushed or brushless DC motor that is controlled by an electroniccontroller consisting of pressure transducers and motor drive circuitry.Typically these pumps can generate up to 350 mmHg vacuum pressure andgenerate a flow of between 2 to 10 litres per minute.

The wound fluid is typically collected in a rigid moulded container thatis made from a suitable injected mouldable grade polymer such as a clearABS that may also be a grade capable of withstanding Gamma radiation.Within the canister a hydrophobic filter is welded or bonded in place.The individual plastic components are ultrasonically or chemicallybonded together to provide an hermetic seal where typically a length ofPVC tube (1 to 2 metres) is bonded into the container assembly. In somecases the entire assembly is sterilised by Gamma irradiation or Ethyleneoxide (EtO) gas and then packaged. This assembly is disposed of asclinical waste once the container has reached its capacity.

Increasingly complex wounds are being treated in the home due to theageing population and pressures on hospitals to discharge patientsearlier. These complex wounds often produce significant levels of woundexudate that are beyond the capacity of an absorbent dressing andrequire an effective means of removing the fluid from the wound site inconjunction with the delivery of a consistent level of pressure at thewound site. Traditional methods of containing wound fluids in a NPWTdevice consists of a rigid plastic moulding manufactured from athermoplastic material such as Acrylonitrile butadiene styrene (ABS) andincorporating a hydrophobic filter to contain fluid within thecontainer. Typically these containers hold between 300 ml and 1000 mland are rectangular in shape and fit into a customised receptacle withinthe pump system enclosure.

The containers are rigid to withstand the vacuum pressure generatedwithin them which can potentially be up to 350 mmHg in a fault conditionand is normally up to 200 mmHg in normal conditions. Conventionalflexible bags such as those used in urine or colostomy collectionapplications have previously not been able to be utilised because theycollapse when subject to the negative pressure thus preventing fluidbeing removed from the wound site and pressure being applied to thewound site.

Typically the vacuum port is at the top of the canister and is protectedby a hydrophobic filter. Depending on the type of system used typicallydetection of a full container is achieved by a non-contact electronicdevice such as a capacitive sensor. Other methods include allowing thecanister to fill with fluids until the hydrophobic filter is occluded;this results in the pressure between the pump and the filter to increasewhile the pressure in the container or the wound dressing decays. Thesystem software detects the pressure differential and interpolates thisas an indication of a full container.

One problem with these methods are that they are prone to prematuredetection of a full container if the unit and the container are tiltedor shaken such as can occur through walking or ordinary movement of apatient. Additionally if the unit is placed on its side or upside downit will trigger the alarm even if there is minimal fluid in thecontainer resulting in the patient or user having to replace thecontainer. In view of the sealed nature of the container it cannot beemptied and needs to be disposed of even if it is only partially filled,this has an economic effect and also an environmental impact.

These issues are not prevalent when the product is used in the statichospital situation but can become a major problem when a patient isdischarged from hospital with a complex wound back into the community.In order to promote wound healing patients are encouraged to beambulatory but hospital style NPWT devices do not support this becauseof the size and weight of the products due in part to container size andcontrol systems and also the need to keep the fluid container in anupright static position.

Another problem present with the use of NPWT in the home concerns thepressure control system. To ensure effective therapy is delivered arelatively constant negative pressure at the wound site needs to bemaintained. If the pressure at the wound site is not accuratelycontrolled then this can lead to a series of problems includinginconsistent wound healing, pain and in extreme cases bleeding. As wellas accurate pressure control there is also a need to maintain a minimumlevel of flow from the wound site. Potentially with a sealed dressing asituation can exist where is there is little or no flow from the woundsite to the wound fluid container.

This can lead to the wound exudate “pooling” at the wound site which canlead to a series of problems including breakdown of the seal of thecovering film to the skin, maceration of the periwound skin area andpotentially an increase in infection at the wound site. Maintainingconsistent pressure and flow at the wound site becomes increasinglyimportant with patients who have been discharged from hospital withcomplex wounds. However increased mobility leads to additionaldifficulties in controlling pressure and flow rates at the wound sitewhich does not always occur in hospital situations when the patients arelying down and relatively immobile. One particular issue is thepotential height difference that can exist between the pump and thewound site, which is often the case when a leg or foot ulcer is beingtreated. This can result in a pressure difference of up to 75 mmHgdepending on the length of tubing used which is due to the hydrostaticpressure present on the wound fluid. If the system is controllingpressure within the pump control unit this can result in a reducedpressure at the wound resulting in reduced therapeutic effectiveness andcompromised wound fluid drainage. Similarly moving the pump unit belowthe wound can in some cases result in a spike of pressure.

Pressure control in traditional NPWT devices is achieved by the use ofpressure transducers measuring pressure within the pump unit andcomparing it with the desired pressure. The pump pressure is thenadjusted via the electronic control system to match the actual pressureat the measurement point to the desired set pressure. Problems with thisarrangement include the discrepancy between the pressure measured at thepump unit and the actual pressure at the wound site. This discrepancycan be caused by the pressure drop across the protective hydrophobicfilter or a height difference between the pump unit and the dressing asdescribed above or the viscosity of the wound fluid. The pressurediscrepancy between the pump unit and the dressing can be compensated byhaving a second pressure transducer sensing pressure at the wound sideof the filter or at the wound dressing itself. However this requirescomplex arrangements of tubing separating the air path from the fluidpath, a second pressure transducer, a safety release solenoid andelectronics to receive and condition the signal from the transducers anda software algorithm to convert this into an output signal that willdrive the pump at the correct level to achieve the target pressure.

In summary user problems that require solving include:

-   -   Providing a means of delivering effective NPWT therapy in the        home (wound fluid removal and accurate pressure control).    -   Allowing NPWT to be administered to ambulatory patients.    -   Remove the requirement for a rigid container thus reducing size        and environmental impact.    -   Providing a means of collecting the fluid so it can be easily        disposed of in the home environment whilst reducing the risk of        contamination and cross infection.    -   Transmission of fluid from the wound site to the dressing is        critical, without this the fluids can collect at the wound        increasing the risk of infection through an increase in Colony        Forming Units (CFU's) and potentially cause maceration of the        peri-wound area.    -   The fluid container needs to function in multi-orientations.    -   The product should be discrete (small and quiet).

Technical issues associated with these are:

-   -   Providing accurate pressure control at the wound without the        need for multiple tubes and sensors to the wound site and the        associated level of complexity in the electronic control system        and software.    -   Providing a means to allow fluid to be drawn away from the wound        at a constant flow preventing the fluid “pooling” at the wound        site.    -   Allowing air flow to be applied at the wound site whilst fluid        is removed into a fluid container and still allow the product to        operate in multi-orientations.    -   Ensure the wound fluid is separated from external contact or        potentially reusable components.    -   Provide a means of collecting fluid in a flexible container        whilst subjecting the container to negative pressure.    -   Enable gas flow through the canister in any orientation.    -   Ensure any filtration means when challenged by the fluid (which        occurs more often in mobile situations) provides a high level of        bacterial retention whilst still able to support a flow rate of        fluid away from the wound.    -   Power levels need to be very low to support the use of small        batteries.    -   The source of negative pressure should be virtually silent.

According to a first aspect of the invention there is provided anapparatus comprising a wound dressing (15) connected to a fluidcontainer (11) via a pump (3), wherein the wound dressing is incommunication with a mechanical pressure control valve (13), the fluidcontainer is provided with an inlet (2) and an outlet (4).

The wound dressing may be a negative pressure wound therapy dressing.The dressing may comprise an absorbent pad (for example a porous pad)and/or a flexible layer (for example a film layer). The absorbent pad inthe wound dressing may be composed of any suitable material, such as forexample a polyurethane reticulated open cell foam and/or gauze (e.g.cotton, polyester/cotton blend, etc.). The flexible layer may be anysuitable film, for example a polymeric film for use in wound dressings,in particular a negative pressure wound therapy dressing, such as forexample a hydrophobic polymeric film.

Suitably, the pressure control valve is connected to the wound dressingvia a tubing connector. The connector may be adhered to the dressing andmay suitably be of separate construction. The connector may include thevacuum tube and the pressure control valve. Alternatively, the connectormay attach the pressure control valve to the tubing between the dressingand the fluid container or vacuum source. In some embodiments of theinvention, the outlet can vent pressure in the container to the externalatmosphere.

According to the present invention, the wound dressing is connected tothe fluid container so as to allow air and fluid to pass from the wounddressing into the fluid container through the action of the pump whichcauses a negative air pressure to be exerted at the side of applicationof the wound dressing. The wound dressing is therefore in gaseouscommunication with the fluid container.

The mechanical (non-electrical) pressure control valve may also bepresent as part of the wound dressing, integrated or embedded within thedressing components e.g. the film, tubing connector or dressing foam orpad. The wound dressing may be suitably connected via tubing to thefluid container. The tubing may be composed of any suitable polymericmaterial, such as for example polyurethane, PVC or silicone. The tubingmay be vacuum tubing or other suitable tubing able to withstand anegative pressure. Alternatively, the wound dressing may be directlyconnected to the pump and/or the pump connected directly to the fluidcontainer.

The fluid container (11) can be rigid or flexible and can be of anysuitable construction for use in connection with the apparatus of theinvention. Semi-rigid containers and flexible bags are also included, asare suitable canisters for retaining fluid.

In embodiments of the invention where the fluid container is flexible,it can be deformed while still retaining its function to contain fluid.The flexible fluid container under negative pressure suitably does notself-occlude and allows fluid or gas flow from the wound dressing. Theflexible fluid container may be composed of any suitable material, forexample a flexible polymeric film, such as polyurethane, PVC etc.

The flexible fluid container will incorporate a means to allow gastransfer between the internal space of the container and atmosphere. Inembodiments in which fluid is pumped into the container then an outletin the form of a vent will be incorporated to allow trapped air toescape to atmosphere thus preventing the container becoming pressurised.This vent may consist of a hydrophobic membrane such as Versapor filtermembrane 3 micron manufactured by Pall Corporation which is bonded inposition by welding or adhesion, the vent function may also be providedby a conventional pressure relief valve.

Common practice in the art prior to the present invention is to userigid containers that are subject to negative pressures.

In some embodiments, the fluid container may also comprise a spacer tomanifold the air and fluid. The spacer fabric may be composed of anysuitable fabric. In certain embodiments, the spacer fabric may be a typeof open-cell foam, suitably in the form of a polymeric material, whichcan have a honeycomb or multi-chambered structure.

In some embodiments of the invention in which the fluid container isrigid, the microporous fluid separator is suitably positioned within thecanister. In alternative embodiments, where the fluid container is aflexible container, the microporous fluid separator may be present as anintegral part of the container rather than a separate part.

The fluid container according to any embodiment of the invention mayalso suitably comprise a fluid absorbing substance (super-absorbentgel), and/or an odour reducing element (for example, an active charcoalfilter), and/or a deodorising substance.

Suitably, the fluid container (11) has an outlet in the form of a valveor vent to act as a microporous fluid separator which allowsmulti-orientation of the container while still allowing gas flow.

The fluid container may be present in the form of a cylinder or asphere. The fluid container may also have a means of indicating when itis full. This may be via a visual indicator on the container itself oran external device which activates when the pump reaches its maximumcapacity.

In the case of a flexible container the expansion of the container isconstrained by a flexible strap, retaining plate or a partly rigidhousing. This is achieved by the container locating on a fixed surfacesuch as the product enclosure and the strap or retaining plate latchingin position. As the container fills with fluid the container is allowedto expand to a pre-determined level because the constraining means i.e.the strap or retaining plate has a limited amount of movement orextensibility, When the limit of the extension is reached the fluidcontainer reacts against the fixed surface that contains a switchmechanism such as a micro-switch or a pressure pad and at apre-determined force this will be triggered signalling the container isfull. In other words, the expansion of the container may trigger aswitch mechanism such as a micro-switch or a pressure pad by reactingagainst a flexible strap, retaining plate or a partly rigid housing.

An example of a flexible strap arrangement for this purpose is a strapmanufactured from Polypropylene webbing, Code No. W19 as supplied byPennine Outdoors. In order to provide some tension to ensure the strapremains in position during use and also holds the container securely anelastic fabric strap 10 to 12 mm in width is stitched or bonded to asection of the webbing strap and pulls the strap tight in position. Thishas the effect of providing a limited amount of extension to the straparrangement and effectively pre-tensions the strap.

Alternatively a strap could be provided that is elastic in nature buthas a pre-determined maximum extended length. Alternatively the strapmay include electrical contacts that make or break a detection circuitwhen the expansion of the container causes the contacts to close oropen. To ensure the pump will not function without the strap orretaining plate in place which potentially may result in the device notdetecting when the canister is full, a sensor may be incorporated intothe strap or retaining plate mechanism to detect that it is fittedcorrectly. An example of a sensor that could be used is a magnetic typeproximity switch such as those produced by Comus International, US.These devices are triggered by the presence of a magnetic field whichcould be provided by a magnet incorporated into the strap or retainingplate, ideally both states could be detected i.e. in position and not inposition, and this could be achieved by a changeover type device orthrough logic within the pump control system. This information couldthen be used to prevent the pump from starting and simultaneouslyadvising the user via an illuminated LED or audible noise that the strapmust be in the correct position.

In the case of a rigid fluid container, a means of detecting when thecanister is full can include a capacitive liquid sensor (examples ofwhich are manufactured by Gill Sensors of the UK and also HoneywellSensing and Control of the USA). Other means include optical sensors anddirect pressure transducers. The output of the chosen sensing means forboth flexible and rigid could be used to send a signal to the pump toswitch off, this signal could also be used to provide a signal to acontrol panel or an alarm indicator top alert the user that the canisterneeds changing.

The pump can be a peristaltic pump, a diaphragm vacuum pump or a rotaryvane pump. The outlet in the container can be a vent or valve. Thecontainer may contain a means of reducing odour, such as an activatedcharcoal filter or a super absorbent gel to solidify the fluid.

In the various aspects of the invention, reference is made to the use ofdifferent pumps. As the skilled person in the art will be aware othersuitable sources of a vacuum could also be used in place of said pumps.Other vacuum sources include but are not limited to: hospital wallsuction devices; portable suction devices; bellows suction devices andsuction devices powered by a spring means.

For example, a peristaltic style pump could be used in place of atraditional air pump. Tubing may be connected from the wound dressingsite and can be fed through the peristaltic pump head, connectingdirectly into the fluid container. A microporous fluid separator (e.g. ahydrophobic filter) can be provided at the outlet of the fluid containerto expel the air drawn through the system. The vacuum level induced atthe wound site can be controlled by the speed of the peristaltic pump.

The invention therefore also provides the use of a positive displacementpump normally intended for pumping fluids and not for generatingnegative pressures.

The mechanical pressure control valve may be a pressure relief valve. Insome embodiments, the mechanical pressure control valve may be a vacuumcontrol valve. The pressure control valve may consist of a springelement and a seal element. The pressure control valve may be orientatedwith the seal element on the outlet port to atmosphere. The pressurecontrol valve may be provided as a “duck-bill” valve.

Where the pressure control valve is a standard pressure control valve itcan be used in the reverse orientation i.e. the normal outlet toatmosphere is connected to the fluid side. Alternatively, the valve canbe designed to operate in this fashion to permit airflow into thedressing.

The invention therefore provides the use of an air pressure controlvalve to act as a check valve when fitted in the reverse orientation andsubjected to fluid pressure on the normal outlet.

The invention therefore includes the use of a standard type pressurecontrol valve originally intended for a pneumatic application in reversewith the intended outlet port orientated on the fluid side of the wounddressing. It is envisaged a custom made vacuum control could also beused.

The fluid container in either the rigid or flexible embodiment containsa microporous fluid separator which in this aspect acts as a vent orvalve. The microporous fluid separator may be a filter. The filter maybe coated in order to provide hydrophobic and/or oleophobic properties.As liquid is drawn off the wound it collects in the fluid container butdoes not exit the container due to the hydrophobic/oleophobic propertiesof the filter and the permanent fixture method to the container materialby the use of adhesives or welding.

The entire system except the pump may be disposable and composed ofsuitable materials that will facilitate disposal, recovery and/orrecycling.

The apparatus therefore provides a closed loop system, in which thepressure control valve acts to control the system without further input.

The various elements of the apparatus such as the wound dressing,tubing, tubing connectors and fluid container described above may besterilised prior to use, or provided in sterile ready-to-use forms,suitably in air-tight blister-packs. Sterilisation may be achieved byany suitable means.

Preferred aspects for the second and subsequent aspects of the inventionare as for the first aspect mutatis mutandis.

According to a second aspect of the invention there is provided anapparatus comprising a wound dressing (15) connected in series to afluid container (11) which is connected in turn through a microporousfluid separator (7) to a pump (3), the fluid container is provided withan inlet and an outlet, wherein the wound dressing is in communicationwith a pressure control valve (13) connected in the reverse orientation.

In this aspect of the invention, the microporous filter is present inthe line between the pump and the fluid container. In one embodiment,the microporous fluid separator will be incorporated into the linebetween the fluid container and the peristaltic pump and a one way valveor air bleed is provided at the end of the tube which vents toatmosphere. Suitably, the fluid separator may be a hydrophobic filter,typically with a pore size of 0.45 microns. The advantage of thisembodiment is that the entire tubing set including dressing and fluidcontainer is isolated from the pump system and therefore any risk ofcontamination of the device is eliminated. The pump is therefore placedat the end of the tube set which draws fluid from the wound dressinginto the fluid container. The fluid container is suitably a flexiblecontainer. The flexible fluid container under negative pressure suitablydoes not self-occlude and allows fluid or gas flow from the wounddressing.

The invention in accordance with the first and second aspects of theinvention therefore provides the use of a flexible bag concept tocontain fluids which is designed to be subjected to negative pressureswith an integrated bacterial barrier membrane.

The fluid container may also comprise a spacer to manifold the air andfluid. The spacer fabric may be composed of any suitable fabric. Incertain embodiments, the spacer fabric may be a type of open-cell foam,suitably in the form of a polymeric material, which can have a honeycombor multi-chambered structure.

In embodiments of the invention in accordance with the first and secondaspects of the invention in which the fluid container is rigid, themicroporous fluid separator is suitably positioned within the canister.In alternative embodiments, where the fluid container is a flexiblecontainer, the microporous fluid separator may be present as an integralpart of the container rather than a separate part.

According to a third aspect of the invention, there is provided a wounddressing comprising an absorbent pad (for example a porous pad), aflexible covering (for example a film covering) and a connection to amechanical pressure control valve in the reverse orientation. Such wounddressings are therefore suitable for use in an apparatus, system ormethod of the present invention as described herein.

According to a fourth aspect of the invention, there is provided amulti-compartment wound fluid container (20) comprising at least twointernal compartments and provided with an outlet (31) and an inlet(19), in which the container comprises a microporous fluid separator(29) which divides the at least two internal compartments, wherein themicroporous fluid separator permits gas flow between the compartmentsand prevents fluid flow to the outlet of the container.

Such containers suitably comprise two compartments. The fluid containermay be rigid or flexible. The construction of the fluid container may beas described above in relation to the other aspects of the invention.The fluid separator suitably acts to prevent egress of fluid from theoutlet of the container.

In one embodiment, the fluid container (20) comprises a microporousfluid separator (29) which is positioned to divide the container whereinthe container is provided with a polymeric material (25, 27) either sideof the fluid separator.

The polymeric material suitably acts as a spacer material in thecontainer and anchors the fluid separator within the container. Thecontainer suitably comprises film layers (21, 23).

In another embodiment, the fluid container (35) comprises a plurality ofmicroporous fluid separators (41) arranged within a housing (39)provided with a plurality of pores (43) which is positioned to engagewith the outlet (37) of the container wherein the container furthercomprises an internal polymeric material (47) arranged at the inlet(49).

The microporous fluid separators may be filters, for example ahydrophobic filter as described herein.

In an embodiment of this aspect of the invention, the fluid containermay be provided with a supported cylindrical microporous fluid separator(e.g. a hydrophobic filter). The support for the separator has an arrayof holes to allow gas communication with the separator (e.g. ahydrophobic filter). This allows for the container to be used on itsside and back without disrupting the function.

In an alternative embodiment, the fluid container (63) is rigid orsubstantially rigid in which the microporous fluid separator (55) isarranged around a housing (57) internally disposed within the lumen ofthe container which is supported by means of a flexible connector (53)to the outlet (51).

The purpose of this is to enable the microporous fluid separator tofloat on any liquid which may be present in the container, thereforeallowing a gas pathway to be present despite the orientation of thecanister. Only when the container is completely full with there be nogas communication.

The microporous fluid separator may be a filter, for example ahydrophobic filter. The flexible means of support (53) can be a coiledtube.

Such containers may be used in conjunction with any aspect of theinvention as defined herein.

According to a fifth aspect of the invention, there is provided a systemfor applying a sub-atmospheric pressure to a wound dressing on apatient, wherein the system comprises an apparatus as defined above inaccordance with any aspect of the invention and a control means. Theapparatus can be controlled via a suitable electric circuit whichoperates the pump.

According to a sixth aspect of the invention, there is provided a methodof treatment of a wound in a patient comprising the steps of applying awound dressing of the present invention to a wound and connecting thewound dressing to fluid container as described herein where the wound iskept under negative air pressure. The negative air pressure can beprovided by connecting the fluid container to a source of a vacuum or asuitable pump.

According to a seventh aspect of the invention, there is provided anapparatus comprising a wound dressing connected to a fluid container viaa pump, wherein the wound dressing is in communication with a mechanicalpressure control valve, the fluid container is provided with an inletand an outlet.

Preferably, the pressure control valve is connected to the wounddressing via a tubing connector. The mechanical pressure control valvemay be part of the wound dressing.

The fluid container may be rigid or flexible. The fluid container outletmay be a gas vent or a valve. The flexible fluid container preferablycomprises at least two layers of film with an integrated vent.

Preferably, the apparatus may comprise a means for detecting the levelof fluid within the flexible fluid container, in which the container isconstrained by a flexible strap with limited extensibility. The means ofdetecting the level of fluid within a wound fluid container may be byoperation of a pressure sensitive switch. Advantageously, the pressuresensitive switch may be a micro-switch. The correct position of theflexible strap may be detected by a proximity switch.

The vent in the flexible fluid container may be a hydrophobic membrane.

The pump may preferably be a peristaltic pump, a diaphragm vacuum pumpor a rotary vane pump.

According to an eighth aspect of the invention, there is provided anapparatus comprising a wound dressing connected in series to a fluidcontainer which is connected in turn through a microporous fluidseparator to a pump, the fluid container is provided with an inlet andan outlet, wherein the wound dressing is in communication with apressure control valve.

According to a ninth aspect of the invention, there is provided aflexible fluid container comprising of at least two layers of film withan integrated vent. The vent may be a hydrophobic membrane.

According to a tenth aspect of the invention, there is provided a wounddressing comprising a porous pad, a flexible covering and a connectionto a mechanical pressure control valve. The connection to the mechanicalpressure control valve may be via a tubing connector. The mechanicalpressure control valve may be integrated into the wound dressingcomponents.

According to an eleventh aspect of the invention, there is provided amulti-compartment wound fluid container comprising at least two internalcompartments and provided with an outlet and an inlet, in which thecontainer comprises a microporous fluid separator which divides the atleast two internal compartments, wherein the microporous fluid separatorpermits gas flow between the compartments and prevents fluid flow to theoutlet of the container.

The fluid container may comprise a microporous fluid separator which maybe positioned to divide the container wherein the container may beprovided with a polymeric material either side of the fluid separator.Alternatively, the fluid container may comprise a plurality ofmicroporous fluid separators arranged within a housing provided with aplurality of pores which may be positioned to engage with the outlet ofthe container wherein the container may further comprise an internalpolymeric material arranged at the inlet.

The fluid container may be rigid or substantially rigid and themicroporous fluid separator may be arranged around a housing internallydisposed within the lumen of the container which may be supported bymeans of a flexible connector to the outlet.

According to a twelfth aspect of the invention, there is provided anapparatus comprising a means for detecting the level of fluid within amulti-compartment wound fluid container in accordance with the eleventhaspect of the invention in which the container is constrained by aflexible strap with limited extensibility. The means of detecting thelevel of fluid within a wound fluid container may be by operation of apressure sensitive switch. The pressure sensitive switch may be amicro-switch. The correct position of the flexible strap may be detectedby a proximity switch.

According to a thirteenth aspect of the invention, there is provided asystem for applying a sub-atmospheric pressure to a wound dressing on apatient, wherein the system comprises an apparatus in accordance withthe seventh, eighth, or twelfth aspect of the invention and a controlmeans.

According to a fourteenth aspect of the invention, there is provided amethod of treatment of a wound in a patient comprising the steps ofapplying a wound dressing in accordance with the tenth aspect of theinvention to a wound and connecting the wound dressing to a fluidcontainer where the wound is kept under negative air pressure.

Examples of apparatus, systems and fluid containers according to theinvention are described further below and in the drawings.

The advantage of the present invention solution is that it addresses thefundamental problems associated with using NPWT treatment which wasoriginally developed for the hospital market for use in the home andcommunity market.

Specifically:

-   -   The present invention provides a means of delivering NPWT        effectively at the wound site in a very simple form without the        need for expensive electronics.    -   The accuracy of the pressure control at the wound is very high        because it is directly controlled.    -   The pressure control means allows the fluid to be constantly        aerated at the wound site allowing the fluid to be effectively        withdrawn from the wound site reducing the risk of “pooling” and        the associated risks i.e. maceration, infection etc.    -   The use of this design of valve i.e. a pressure control valve in        reverse provides an effective means of sealing the dressing        against fluid leakage when the negative pressure is removed.    -   The means of controlling pressure at the wound site allows        alternative pump type systems to be used such as Positive        Displacement Pumps which include peristaltic pumps. Previously        these could not be used without wound site pressure control        because the sealed nature of these systems means that pressure        at the wound site cannot be controlled by reducing speed of the        pump as is the case with conventional air pumps.    -   The direct pressure control allows other vacuum sources to be        used that previously could not be used because they require        regulation at source, this includes wall suction or potentially        a mechanical means such as bellows or sprung loaded vacuum        generation device. Provided the vacuum source is higher than the        required wound site target pressure then the pressure control        valve at the dressing will maintain the correct pressure at the        wound.    -   The invention allows the use of a flexible fluid container; this        reduces material cost, size and weight. User acceptance is        higher because it is more conformable and the concept of wearing        a flexible fluid container is also accepted in the homecare        environment e.g. urinary and colostomy bags.    -   The invention solution can utilise procedures that are already        in place for the disposal of these types of flexible bags thus        reducing the environmental impact over the incineration of a        rigid moulded container.    -   The invention allows the product to operate through a range of        orientations whilst subject to movement and vibrations and still        adequately fill the container with wound fluid.    -   The pressure can be maintained at the wound site without the        complexity of multiple pressure sensors, signal conditioning        circuitry and pressure control algorithms.    -   Overall cost, size and weight are reduced.    -   The direct nature of the pressure control at the wound site        results in a very efficient use of the vacuum source because the        rest of the system can be air tight resulting in very low noise        and very low power consumption.

In the application reference is made to a number of drawings in which:

FIG. 1a and FIG. 1b show systems of the invention

FIG. 2 shows a system of the invention

FIG. 3 shows a flexible fluid container of the invention.

FIG. 4 shows a flexible fluid container of the invention

FIG. 5 shows an alternative embodiment of a flexible canister of theinvention

FIG. 6 shows an alternative embodiment of a flexible canister of theinvention

FIG. 7 shows an alternative embodiment of a rigid canister of theinvention

FIG. 8 shows an isometric view of an embodiment of an apparatus of theinvention

The invention will now be further described in detail with reference tothe following Figures and Examples which are not to be construed asbeing limitations to the invention.

FIG. 1a and FIG. 1b show systems of the invention whereby a wounddressing is connected to a fluid container (11) via tubing (5) which issuitable for use with a peristaltic pump. Connected in proximity to thewound dressing is a pressure control valve (13) which limits thenegative pressure produced at the wound site to a predetermined value.The fluid container (11) can either be rigid or may be flexible to allowconformity to the patient (this is described in further detail in FIG. 3below). The container has an inlet (2) and an outlet (4). Between thecontainer (11) and wound dressing (15) a peristaltic pump (3) is used.The tubing (5) is connected through the peristaltic pump. The containerhas a means to allow the gas to escape such as a vent or valve (1)positioned at the outlet (i.e. an air control valve), thereby preventingover inflation of the container in its flexible form and would typicallyincorporate a hydrophobic filter to ensure the fluid is contained. Thecontainer could also contain a means of reducing odour such as anactivated charcoal filter or a superabsorbent gel to solidify the fluid.An example of a suitable style of container for this purpose is a 540 mlvented urinary bag such as that manufactured by Hollister Inc., USA.

FIG. 1a shows one embodiment of the invention of a system in which theoutlet (4) comprises a vent suitably composed of a gas permeablehydrophobic membrane as described herein. FIG. 1b shows an alternativeembodiment in which the outlet (4) comprises an air control valve (1) asdescribed herein.

FIG. 2 shows a wound dressing (15) connected via tubing to a container(11) where the wound dressing is provided with a negative pressurecontrol valve (13). The container (11) is connected to a hydrophobicfilter (7) via tubing (9). The filter is connected to an air controlvalve (1) by tubing (5) that is suitable for use with a peristaltic pump(3). The container (11) can either be rigid with a hydrophobic filterintegrated within, or a flexible container as described in FIGS. 3 and4. In the latter two embodiments, the hydrophobic filter (7) is nolonger present externally since a corresponding filter is present withinthe container.

A pressure control valve (13) as shown in FIGS. 1 and 2 is positioned inreverse to its normal orientation between the fluid container (11) andthe wound site dressing (15) to provide better pressure control, due tothe closer proximity to the area requiring regulated pressure andtherefore is not prone to the hydrostatic head effect caused by pullingfluid upwards. Additionally by placing the pressure control mechanism onthe wound side of the hydrophobic filter then the effect of the pressuredrop across the filter is eliminated. Typically the pressure controlvalve would be set at a pre-determined pressure such as 2.5 PSI (129mmHg) with a 5 to 15% crack tolerance. A standard arrangement for thistype of valve is 3 ports with the control element consisting of apolymeric seal such a silicone and a stainless steel spring inside aPolypropylene or similar injection moulded body.

An example of this is available from Qosina Part No. D002501. A largerange of alternative valve arrangements could be used including DuckBill style valves orientated in a reverse configuration i.e. with thevalve seal lips on the fluid side. Another alternative is an Umbrellastyle control valve. A traditional style ball and spring valve couldalso be used. It is envisaged a purpose made vacuum valve could also beused. Typically a range of dressings will be available for differingwounds such as leg and pressure ulcers and the dressings will be matchedin size and pressure settings to accommodate this. Additionally valveswill be available that can be adjusted by the user and may be situatedat various positions from the dressing to the negative pressure source.

One method of negative pressure generation is by a peristaltic pump (3)as shown in FIGS. 1 and 2. This pump allows for a combined, disposableset of components to be used. No fluid comes into contact with the pumpand this omits the requirement for protective filter systems for thepump. With the use of the pressure control valve (13) that limits thepressure at the wound site, the control system for the pump is limited.A potential peristaltic pump could be the 400F/A Single ChannelPrecision Pump by Watson Marlow Alitea. Other types of positivedisplacement pumps could be used and integrated into the tubing set suchas a disposable pump head, an example of this type of pump is theCAPIOX® Disposable Centrifugal Pump manufactured by Terumo, USA. Anotherexample of a pump that could be used is a Kamoer KPP Peristaltic dosingpump that has additional advantages of a small size and low powerrequirements.

An alternative to a peristaltic pump is a small diaphragm vacuum pumpwith a flow rate of between 1.5 litres and 2 litres per min at free flowwith a maximum vacuum of 370 mmHg an example of this would a pumpmanufactured by KNF Neuberger GmbH of Frieburg, Germany Model numberNMS020L. A range of other vacuum pumps could be utilised with a range offlow rates up to 10 litres per min if required. For the communityapplication minimum user controls are required so normally the devicewould be pre-set at a vacuum level slightly above the required woundsite pressure level to account for variances in pressure due to heightdifferences etc. In practice this will result in a small constant flowof air at the wound site which will ensure there is mobility of thewound fluid from the dressing to the container.

FIGS. 3 and 4 show flexible fluid containers of the invention. Thehydrophobic filter (29) is encapsulated between the two films (21, 23)that form the container and this effectively produces a wet side and dryside over the entire area of the filter. Sections of a spacer material(25, 27) prevent the film collapsing and occluding the filter and alsoprovide a means of manifolding the fluid evenly within the container.Because the filter covers the entire area of the container in anyorientation a section of the filter will be open until the containerfills to its full capacity. The filter surface is treated to ensure itis both Hydrophobic and Oleophobic and therefore will resist wetting byeither water based liquids or fats and lipids. This means that splashingof fluids will bead on the surface and not spread over the surface ofthe filter. The super absorbent gel also immobilises the fluids andprevents splashing.

The fluid container consists of two layers of Polyurethane film (21, 23)such as that manufactured by Chorino Grade UE80 or Epurex Platilon GradeU073 manufactured by Bayer. PVC material could also be used, thematerial used in the construction of blood bags is particularlysuitable, an example of this type of material is Renolit SolmedTransufol Seta 3224 manufactured by Renolit, located in the Netherlands.Within these two layers a hydrophobic filter (29) such as Versaporfilter membrane 0.8 micron manufactured by Pall Corporation isencapsulated by RF welding, Ultrasonically welding, heat impulse weldingor bonding to the film layers. Alternative membrane pore sizes could beused ranging from 0.2 micron to 10 micron could be used. Either side ofthis filter is sections of a spacer material (25, 27)); an example ofthis is manufactured by Mueller textiles of Germany, Grade 5754. Anotherexample is Stimulite® manufactured by Supracor® of USA, which is aflexible bonded honeycomb polymer which provides resistance in one planebut allows flexibility in the others. Additionally other materials maybe added such as an active carbon filter to reduce odour or asuper-absorbent gel such as a Sodium Polyacrylate composition topartially solidify the fluid (this would be incorporated into 27).Connected to the flexible container assembly an inlet (19) and outlet(31) tube or port is hermetically joined by RF welding, UV or solventbonding or by a similar process.

The outlet tube or port is connected to a negative pressure air source(indicated by air flow direction arrow (33)) that evacuates the airwithin the system and at the wound site dressing. Typically thepressures will vary between 25 mmHg and 200 mmHg.

The inlet tube (19) is connected to a wound dressing (and air flowdirection arrow (17) shows the flow of liquid from the wound into thedressing). Various volumes of the flexible container can be producedaccording to the required clinical application but for the homecareapplication then this typically would be 100 ml. Utilising thisprinciple, containers with different capacities could be producedbetween 50 ml and 5 litres.

FIG. 5 shows an end section of an alternative flexible container. FIG. 6shows a front section of the fluid container (35) is a flexible sealedcontainer of a PU film which is RF Welded together. A fluid inlet (49)is bonded into the film. A vacuum source outlet (37) is fitted into thefluid container at (45). Within the sealed fluid container is aflexible, non-compressible material (47) that prevents the fluidcontainer from self-sealing, thereby preventing fluid to be drawn intothe fluid container. The vacuum source tube (37) fits into a cylindricalplastic component called the filter housing (39). This has a series ofribs along the length that prevents the film of the fluid container fromadhering to the hydrophobic filters (41) when subjected to negativepressure. Holes (43) in the housing (39) allow gas to pass between theinterior of the filter housing and the space around the housing. Thebenefit of this design is to allow for a multidirectional fluidcontainer whereby only until the canister is completely full will thefilter membrane be occluded which will prevent further fluid uptake.

FIG. 7 shows a cross sectional front view of a rigid fluid container(63) with fluid inlet port (59) and outlet port (51). The outlet portpasses through (63) and is fixed at the intersection. The outlet port(51) is connected to a coiled, flexible tube (53). This is fixed to ahollow sphere (53) which has a plurality of holes allowing gascommunication between the hollow interior of the sphere and the fluidcontainer. Surrounding the sphere (57) is a hydrophobic filter (55)which covers the holes and prevents fluid passing into the sphere butallows gas to pass from the inlet (59) through the sphere and throughthe outlet (51) to a vacuum source. When fluid (61) is present in thefluid container, the sphere is allowed to float on the level at anyorientation whilst still being connected via the coiled tubing (53).

The embodiment as shown in the drawings has been prototyped and testedon the bench. This proved that pressures could be maintained within asmall tolerance at the wound site utilising a mechanical valvearrangement over a range of conditions which simulated real clinicalconditions. Additionally it was proved that a flexible fluid containercould be produced with a hydrophobic filter barrier. The prototype wasable to function under 200 mmHg of negative pressure and uptake fluidinto the fluid chamber. No fluid passed through the filter barrier. Thefollowing test results show the invention in practice.

FIG. 8 shows an isometric view of an apparatus of the invention in whicha wound dressing (15) is in communication with a mechanical pressurecontrol valve (13). The wound dressing is connected via tubing (5) to afluid container (11) via a pump (3).

EXAMPLES OF THE INVENTION

Testing was carried out utilising the arrangement as described inFIG. 1. The test equipment and components used were as follows:

-   -   Watson Marlow 102U Bench top Peristaltic Pump    -   Watson Marlow Peristaltic tubing Pumpsil 913A (4.8 mm bore×1.6        mm Wall)    -   Test dressing (100 mm×50 mm×30 mm) 150 cc Volume    -   Pressure control Valve. Qosina D002501. 2.5 PSI Cracking        pressure+1-15%    -   Manual Vacuum Gauge. SM Gauge. 1.6% Accuracy over Full Scale        Deflection.

Test 1 (Pump Between Dressing and Fluid Container) See FIG. 1

Test 1a: Closed system, no pressure control valve Pump Fluid Pressurespeed flow at dressing Comment 200 RPM  0 >350 mmHg Closed system 30 RPM0 >200 mmHg 30 RPM to   200 mmHg Pressure not reduced by reducing 5 RPMpump speed. System needs to be opened to achieve pressure decay

Test 1b: Pressure relieve valve (inverted) placed at dressing Pump FluidPressure speed flow at dressing Comment  30 RPM 0 120-125 mmHg Reached150 mmHg (cracking pressure) in 60 seconds stabilised to 120 mmHg 200RPM 0 120-130 mmHg Valve compensated for increased flow to maintainconstant pressure

Test 1c: Fluid introduced, Pressure valve fitted Pump speed set tomaintain a constant fluid flow Pump Fluid Pressure speed flow atdressing Comment 30 RPM 12.5 ml/hour 120-125 mmHg 30 rpm maintainsconstant flow rate

Test 1d: Height difference introduced (dressing set at 0.5 metres belowthe pump unit) Pump speed Fluid flow Pressure at dressing Comment 30 RPM12.5 ml/hour 120-125 mmHg No pressure drop off due to height difference

Test 1e: Overnight test using realistic wound exudate flow rates Pumpspeed Fluid flow Pressure at dressing Comment 5 RPM 2.25 ml/hour 120-125mmHg Test run for 20 hours, 45 ml fluid removal

Test 2: Inline container test. See FIG. 2 Pump Pressure speed Fluid flowat dressing Comment 62 RPM 210 120 mmHg Rapid filling test, 14 minutesto full ml/hour canister (50 ml). Pressure maintained at dressing side

Conclusion of Testing

Test 1a). With a closed system (air only) utilising a peristaltic pumpit was demonstrated that the pressure could not be controlledadequately, as the pump speed increased the pressure correspondinglyincreased to in excess of 350 mmHg. Decreasing the pump speed did noteffectively reduce the pressure and it remained at over 200 mmHg even atthe lowest setting used of 5 RPM. Any pressure decrease was only due toconnector leakages and the breathability of the drape.

1b). With the pressure valve fitted at the dressing site at full speedof 200 mmHg the wound site pressure was limited to 120 mmHg+/−2.5 mmHgwith a momentary maximum of 150 mmHg as the valve initially opened.

1c). With fluid introduced at 30 rpm a constant flow rate was achieved.The fluid flow was aided by small amounts of air being drawn through thevalve which allowed mobility of the fluid from the dressing to thecontainer.

1d). Changing the height of the dressing relative to the pump (0.5metres) did not result in any measurable pressure change at the website.

1e). A longer duration test (20 hrs) showed over an extended period alow level of fluid (2.25 ml per hour) was withdrawn at a constant ratewithout any issues or alterations in parameters, the flow rate was setat a very low flow rate (5 rpm) which results in very low noise levelsand power consumption.

This flow rate is equable to certain types of leg and foot ulcers.

Test 2). The inline container configuration was tested at a relativehigh flow rate 210 ml/hour to stress the filter. The container filled tocapacity, maintained the target pressure and the filter was notbreached.

Results of the Testing:

A standard pressure control valve was used in the reverse orientationi.e. the normal outlet to atmosphere was connected to the fluid side,the variance of negative pressure readings was well within the statedmanufacturers tolerance of +/−15%, which would normally relate to atotal tolerance of 38 mmHg at the normal working pressure. The pressuresmeasured after the valve originally opened and the pressure stabilisedto be in the order of 10 mmHg total working tolerance.

This is believed to be significantly more accurate than electroniccontrol systems that rely on multiple conduit pathways and multipleelectronic components.

The introduction of the pressure valve had a second effect beyondpressure control that was not anticipated, this was to allow theintroduction of small amounts of air into the system at the dressingsite. This had two effects, the first was to allow constant flow offluid from the dressing at a very low flow rates, the second was toprovide a mechanism to reduce pressure at the wound site when sealedpump systems such as a peristaltic pump is used. Additionally the valvehad the effect of aerating the fluid evenly causing mobility whichappeared to be different in nature when a basic leak is introducedthrough an orifice. One explanation for this may be due to the design ofthe valve and the characteristics of the sprung loaded component andseat, although this valve is designed to relieve positive air pressureit has an advantageous effect in regulating air inflow under negativepressure when fitted in reverse. A second major advantage of the valvearrangement is due to the reverse nature of the sprung loaded actionwhen fluid is forced back into the dressing the valve will be forcedclosed effectively sealing the dressing. Several scenarios exist whenthis can happen; one example is when therapy is paused for when thepatient is taken a shower, in this case gravity or pressure against thedressing could cause fluid to pool in the dressing. Normally if thedressing contained any passage to atmosphere then fluid could leak outcausing an infection risk.

In the case of devices that contain sensing tubes or conduits to thecontrol unit to control pressure these can potentially fill with fluidwhen negative pressure is paused that may cause blockages, thissituation is eliminated in the present invention.

What is claimed is:
 1. An apparatus comprising a wound dressing, a fluidcontainer having an inlet and an outlet, tubing which interconnects thewound dressing and the fluid container and which forms a fluid flow paththerebetween, a filter on the fluid flow path downstream of the inlet, aperistaltic pump which accommodates said tubing so as to be isolatedfrom the fluid flow path; and a mechanically automatic pressure controlvalve on the fluid flow path interposed between the wound dressing andthe peristaltic pump to encourage flow away from the wound site, whereinthe mechanically automatic pressure control valve is upstream of thefilter.
 2. The apparatus as claimed in claim 1, wherein saidmechanically automatic pressure control valve comprises a port toatmosphere which is set to open at a predetermined pressure to maintaina constant pressure by permitting airflow into the tubing.
 3. Theapparatus as claimed in claim 2, wherein said mechanically automaticpressure control valve further comprises a seal downstream of the portand a spring downstream of the seal.
 4. The apparatus as claimed inclaim 3, wherein the port, seal and spring are spaced away from thefluid flow path to prevent blockage whilst regulating air flow throughthe mechanically automatic pressure control valve to the tubing.
 5. Theapparatus as claimed in claim 1, wherein the fluid container isflexible.
 6. The apparatus as claimed in claim 1, wherein the wounddressing has a porous layer covered by a flexible film layer enablingdirect-contact wound compression.
 7. The apparatus as claimed in claim1, wherein the filter has at least one of hydrophobic and oleophobicproperties to prevent or inhibit liquid passing but allowing gas topass.
 8. The apparatus as claimed in claim 1, wherein the flier isencapsulated by the fluid container to prevent undue expansion of thefluid container.
 9. The apparatus as claimed in claim 1, wherein theperistaltic pump is separate and upstream of the fluid container. 10.The apparatus as claimed in claim 1, In which the pressure control valveis one of: a check valve and a duck bill valve.
 11. The apparatus asclaimed in claim 1, in which the outlet of the fluid container is a gasvent.
 12. The apparatus as claimed in claim 1, in which the outlet ofthe fluid container is a valve.
 13. The apparatus as claimed in claim 1,in which the fluid container comprises at least two layers of apolymeric film and a vent which comprises the filter.
 14. The apparatusas claimed in claim 1, further comprising a pressure pad or pressuresensitive switch for detecting the level of fluid within the fluidcontainer, in which the container is constrained by a flexible strap.15. The apparatus as claimed in claim 1, further comprising a pressuresensitive switch for detecting the level of fluid within the fluidcontainer.
 16. The apparatus as claimed in claim 15, in which thepressure sensitive switch is a micro-switch.
 17. The apparatus asclaimed in claim 14 in which the flexible strap has a correct position,wherein the correct position of the flexible strap is detected by aproximity switch.
 18. A system for applying a sub-atmospheric pressureto a wound dressing on a patient, wherein the system comprises anapparatus as claimed in claim 1 and an electric circuit which operatesthe pump.
 19. An apparatus comprising a wound dressing, a fluidcontainer having an inlet and an outlet, tubing which interconnects thewound dressing and the fluid container and which forms a fluid flow paththerebetween, a filter on the fluid flow path downstream of the inlet, aperistaltic pump which accommodates said tubing so as to be isolatedfrom the fluid flow path; and a mechanically automatic pressure controlvalve on the fluid flow path, wherein the mechanically automaticpressure control valve has a port to atmosphere which is set to open ata predetermined pressure to maintain a constant pressure by permittingairflow into the tubing, a seal downstream of the port, and a springdownstream of the seal.
 20. The apparatus as claimed in claim 19,wherein the port, seal and spring of the mechanically automatic pressurecontrol valve are spaced away from the said fluid flow path to preventblockage whilst regulating air flow through the valve outlet to thetubing.